The light-sensitive emulsion layer in a silver halide light-sensitive material comprises a coated silver halide emulsion, which is imagewise exposed and then developed to be reduced to black metallic silver. At this time, in a conventional negative film, more metallic silver is produced to provide a high optical density in an area in which an amount of light for exposure is larger, and less metallic silver is produced to provide a low optical density in an area in which an amount of light for exposure is smaller (a reverse relation exists in the case of a positive film). A smaller amount of silver is necessary to obtain the same optical density when the covering power of a silver halide emulsion is improved. Covering power is defined as a value obtained by dividing the optical density by the amount (g) of developed silver per unit area. This definition is described in T. H. James, The Theory of the Photographic Process, 4th Ed., 1977, page 405 (Macmillan Publishing Co., Inc.). It is easily understood from the above definition that the higher is the covering power, the smaller is the amount of silver necessary to obtain the same optical density. Silver is an expensive metal, and therefore the necessary amount of silver can be reduced to lower the cost of production of a silver halide light-sensitive metal by increasing the covering power. Also, there is a possibility of an exhaustion of silver in future, and, therefore, it is very worth in a saving of silver to increase the covering power.
Accordingly, there have been many attempts to increase covering power. One method is to increase the covering power by adding a natural or synthetic hydrophilic polymer to a silver halide emulsion layer. Other known methods of increasing covering power include adding, for example, hydroxyethyl cellulose as described in U.S. Pat. No. 3,003,878, poly-N-vinyl lactam as described in U.S. Pat. No. 3,058,826, polyacrylamide as described in U.S. Pat. No. 3,271,158, laminarin as described in Belgian Pat. No. 611,622, mannan as described in Belgian Pat. No. 611,623, or polyacrylic acid or polymethacrylic acid as described in Japanese Patent Application (OPI) No. 182732/82 (the term "OPI" as used herein refers to an unexamined published patent application).
However, when these natural or synthetic hydrophilic polymers are added to a silver halide emulsion, there were problems, such as that the covering power is improved but not enough, that a so-called "interlayer adhesion" between a silver halide emulsion layer to which the above polymers are added and an adjacent layer thereto lowers, and that the film strength of a silver halide emulsion layer to which the above polymers are added lowers markedly.
For example, when polyacrylamide as described in U.S. Pat. No. 3,271,158 of polyacrylic acid or polymethacrylic acid as described in Japanese Patent Application (OPI) No. 182732/82 is added to a silver halide emulsion layer of a silver halide photographic material, there is a problem that a covering power thereof is improved, but the interlayer adhesion between the emulsion layer and the adjacent layer is lowered. When a silver halide photographic material having the above emulsion layer is developed, there is another problem in that component layers of the silver halide photographic material containing the above emulsion layer are easy to flaw because of remarkable lowering of a film-strength of the above emulsion layer. Accordingly, it was difficult to produce a silver halide light-sensitive material having a high covering power by adding these polymers into an emulsion layer.
Also, as a photographic light-sensitive material generally comprises an insulating support and photographic layer, electrostatic charge is frequently accumulated by contact friction or separation between surfaces of substances of the same or different kinds during the production of photographic light-sensitive materials or at the time of use. This accumulated electrostatic charge causes various problems. The most serious problem is that a light-sensitive emulsion layer may be exposed by discharge of accumulated electrostatic charge before development, whereby a spot or a line (often having a tree branch or a feather-like appearance) occurs on the developed photographic film. This is called "a static mark" and impairs a value of merchandise of a photographic film remarkably. In certain cases, the value of merchandise is quite gone. Static marks induced by accumulated electrostatic charge tend to increase significantly when increasing the speed of a photographic light-sensitive material and the speed of processing a photographic light-sensitive material. Recently, static marks have more easily arisen, because photographic light-sensitive materials have been more subject to high speed and severe treatment, resulting from a high-speed coating, a high-speed photographing, and a high-speed automatic processing.
It is preferred to add an antistatic agent to a photographic light-sensitive material in order to decrease the chances of damage arising from electrostatic charge. However, all antistatic agents generally used in the other field cannot be used in a photographic light-sensitive material, since there is a specific limitation of a photographic light-sensitive material. That is, an antistatic agent capable of being used in a photographic light-sensitive needs to have the properties of (1) not having an adverse influence on any of the photographic properties such as sensitivity, fog, granurality, sharpness, (2) not having an adverse influence on the strength of a layer (that is, not becoming more easily damaged by rubbing or scratching), (3) not having an adverse influence on the antiadhesive characteristics (that is, not becoming more easily adhered to the surface of another photographic light-sensitive material and to the surface of another substance), (4) not accelerating the reduction of the power of the processing solution, and (5) not lowering the strength of adhesion between two component layers in a photographic light-sensitive material. Accordingly, there are a number of limitations when applying an antistatic agent to a photographic light-sensitive material.
One method to avoid damage arising from electrostatic charge is to remove the electrostatic charge in a short time before a static mark-causing discharge occurs, by raising the electric conductivity of a surface of a light-sensitive material.
Accordingly, methods to raise the electric conductivity of a support or a surface coated layer of a photographic light-sensitive material have been investigated. One method was to use an ionic polymer. An attempt to use an anionic polymer having an carboxyl group in order to prevent charging with electricity in a photographic light-sensitive material is disclosed in Japanese Patent Publication Nos. 53587/82, 15375/82, German Patent No. 1,745,061, Japanese Patent Publication Nos. 23827/74, 14415/80, 15267/80, Japanese Patent Application (OPI) No. 89979/73, U.S. Pat. Nos. 2,279,410, 3,791,831 and Japanese Patent Publication No. 28937/72. However, these polymers are hydrophilic, and, when a silver halide photographic light-sensitive material containing these polymers is processed for development, there are problems in that these polymers dissolve out into the (aqueous) developing solution to accumulate therein, whereby the thereafter developed silver halide photographic light-sensitive material is stained and a minute mark of elusion occurs to make a silver halide light-sensitive material cloudy. Moreover, these polymers have a problem in that the power to prevent charging with electricity decreases remarkably, because these polymers diffuse from the layer to which they are added to other layers. An attempt to make a polymer of acrylic acid or methacrylic acid a cross-linked latex in order to solve these problems is described in U.S. Pat. No. 4,301,240. This cross-linked polymer of acrylic acid or methacrylic acid can solve the problem of the above-described non-cross-linked polymer having a carboxyl group. This cross-linked polymer of acrylic acid or methacrylic acid has a power enough to prevent charging with electricity in usual or high humidity conditions, but in low humidity conditions, it has insufficient power to prevent charging, whereby it is substantially impossible to prevent the occurrence of a static mark in a photographic light-sensitive material.